No specific name requirement on the constructor for the external interface, or for the implementation (Class::Singleton requires clients use instance(), and that subclasses override _new_instance()). With aspects, you can change the cardinality of your objects without changing the clients, or the objects themselves.

No need to inherit from anything- use pointcuts to specify the constructors you want to memoize. Instead of pulling singleton behavior from a base class, you are pushing it in, using the aspect.